We often get questions about which TV display technology is "better". In
particular, the question arises in regard to DLP vs. LCD. This article
hopefully will provide some answers.

DLP
(Digital Light Processing - Texas Instruments) technology offers several
potential advantages over LCD (Liquid Crystal Display - marketed by many
companies with trademarked names and variations, such as JVC's D-ILA and
Hitachi's LCOS, both of which are reflective LCD rather than conventional
transmissive LCD). In particular, DLP projectors have a much higher (higher is better) pixel “fill
factor” than transmissive LCD, that is more of the area of an individual pixel is used for the
picture as opposed to the grid surrounding the pixel (D-ILA and LCOS do have an excellent fill factor though). With all digital or
fixed panel projectors (non CRT), the grid between the individual pixels is
static, wasted space, and this can be visible to viewers. It is most commonly referred to as the “Screen Door Effect",
or SDE, and many viewers find SDE to be objectionable when it is noticeable.
Because of its higher fill factor (pixels closer together, so less space in
between pixels), DLP-based projectors generally produce a much smoother
looking image with less SDE compared to LCD-based projectors of similar
resolution.

Below is a
diagram showing pixels arranged with a low fill factor and a high fill
factor. The pixels are the same size, but are closer together in a high fill
factor panel with the same resolution. With the high fill factor, you cannot see the regions between the
pixels as easily, so it has less of a Screen Door Effect.

The second
advantage often identified with DLP-based projectors is a high contrast
ratio and efficient use of light. Contrast ratio is figure of merit that
compares the ratio between the brightest white and the darkest black that a
projector can produce. The light output of the projector in lumens and the
contrast ratio are generally considered two of the most important
performance indicators for HT usage. Generally, a better contrast ratio
leads to better black levels and, therefore, a more realistic presentation
of darker scenes in movies and TV programs.

While DLP technology has several significant advantages over competing
technologies such as LCD, it also has several potential drawbacks,
especially in a single chip design like most of them are right now, based on
the way color and shades of gray are produced. As mentioned above, when the
DLP mirrors flip they can turn light on or off (1 or 0). Video images
consist of many different shades of gray or luminosity and, more often than
not, also have color. To make the many shades of gray, or regions of varying
light intensity, the mirrors have to flip on and off rapidly so that the eye
(or the brain) averages or integrates the image and perceives the desired
light level. This is called dithering. Other forms of averaging or dithering
may also be applied to regions of the screen to further extend the number of
levels of gray that can be perceived by the viewer.

With “single-chip” DLP-based projectors, the perception of color is created
in a similar way to how the levels of gray are made. To make colored images,
light from the bulb passes through a “Color Wheel,” a small rapidly rotating
wheel with red, green, and blue windows or colored filters, before it hits
the DLP chip. (Color wheels on some single-chip DLP-based projectors also
have a clear or white section.) To make red, the light from the chip is
turned on (the mirrors flip on in the red part of the image) when the red
window in the wheel is aligned with the light path, etc. Sequentially
turning on the red and blue for a given pixel makes purple, red and green
make yellow, etc. Thus by rapidly flipping the mirrors in synchronization
with the color wheel, the projector can make all the colors and shades of
gray needed for video images. While it is hard to imagine many tiny mirrors
flipping so quickly to make a moving image, the system obviously works, and
DLP-based projectors are among the most popular and well thought of models
available today. For a very informative explanation of how DLP works visit
http://www.dlp.com and take the demo.

The potential drawback of this single-chip DLP technology is that in any
given instant, the picture on the screen is not the total image, but is
instead rapidly alternating between images consisting of the individual red,
green, and blue colors. Thus the eye and the brain play the last critical
role in making single chip DLP projectors work, by combining or averaging or
integrating the picture, so that the viewer perceives the desired image and
not the rapidly flashing momentary components of the image.

With a static picture from a single-chip DLP-based projector, it is easy to
understand how this averaging works just fine. Where things potentially
start to fail is when there is motion in the image, or when one blinks or
rapidly moves one’s eyes quickly between various parts of the image. In
these cases, the perceptual integration of the image may break down and one
might see “rainbows” or false flashes of color, in the image. Some
individuals have also reported getting headaches after watching single-chip
DLP projectors for any length of time. It appears that not all individuals
handle this color averaging process equally well. Newer single-chip DLP-based
projectors use a higher speed (5x) six segment color wheel, thus greatly
reducing the likelihood that an individual will perceive these artifacts.
Also, Double Data Rate (DDR) DMD chips reduce artifacts even further by
updating the information going to each mirror at a faster rate.

Below is
an animated graphic showing what the rainbow effect looks like. You would
see it in high contrast areas of the picture when you move your eyes up and
down or side to side rapidly.

In
contrast, three-chip projectors use a separate chip for the red, green, and
blue colors, and thus, simultaneously present the RGB images so that no
temporal averaging or integration by the user is necessary. Today, virtually
all LCD based projectors intended for HT usage are three-chip projectors.
There are some three-chip DLP projectors out there, but these are very
expensive and are typically for commercial use only. Hopefully this will be
changing.

Thus in today’s HT projector market one can select DLP-based projectors with
their better contrast ratios, better black levels, and smoother image, but
with the risk that some individuals may see artifacts that are not present
with LCD projectors.